Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

COLON AND RECTUM

90%, respectively, for internal openings (218). Even when using a low field (0.1-T) MR unit, results have agreed with the final diagnosis in over 95% (219). Nevertheless, some MR studies are more pessimistic when evaluating the site and extent of a fistula; for instance, in patients with subsequently confirmed fistula-in-ano, MRI detected only 42% to 50%, depending on the radiologist’s experience (220).

Genitourinary Tract Fistulas

Renocolic Fistula

Renocolic fistulas are usually secondary to renal inflammation or neoplasms. An occasional patient with xanthogranulomatous pyelonephritis and ureteric obstruction develops a renocolic fistula. An antegrade or retrograde pyelogram should identify these fistulas. Computed tomography usually reveals a complex air-fluid collection within either the kidney or the adjacent soft tissues.

Urethrorectal Fistula

Rare urethrorectal fistulas consist of fistulas communicating between the prostate or bulbomembranous urethra and rectum. Trauma from missiles is a not uncommon cause of these fistulas. A number of these patients have had prior surgery or complex anoperineal suppuration.

Detection of urethrorectal fistulas is straightforward, either via a urethrogram or a contrast enema.

Some of these fistulas close spontaneously after a more proximal-sigmoid colostomy and suprapubic cystostomy; others require surgical correction.

Colorectal Vesical Fistula

Most enterovesical fistulas are secondary to inflammatory bowel disease or diverticulitis, with an occasional one originating from a colon carcinoma, bladder carcinoma, or other neoplasms. Pneumaturia is common but not universal in patients with a colovesical fistula.At times cystitis is the primary presentation.

Most colovesical fistulas can be identified by barium enema, cystography, or cystoscopy. In some patients a one-way check valve mecha-

Figure 5.43. Rectovaginal fistula (arrow) secondary to lymphomatous infiltration.

nism presumably exists, and in any one patient not all three studies identify a fistula.

Rectovaginal Fistula

Most rectovaginal fistulas are secondary to birth trauma, gynecologic surgery, or pelvic radiation. A rare cause is pelvic amebiasis or actinomycosis. Diverticulitis predominates as a cause of colovaginal fistulas.

Either a barium enema or vaginogram identifies these fistulas (Fig. 5.43). Pelvic MRI is useful to define involved tissue planes. T2weighted images identify rectovaginal fistulas as hyperintense linear defects. Most internal opening can be identified.

A nitinol-silicone double-disc device was inserted transrectally into a rectovaginal fistula and the fistula occluded (221). Such an occluding device appears useful in a setting of tumor, pelvic radiation, and reluctance for repeat surgery in someone with limited life expectancy.

Other Fistulas

Some iatrogenic gastrocolic fistulas are created due to inadvertent transverse colon puncture during percutaneous gastrostomy. An occa-

sional duodenocolic fistula is secondary to colonic Crohn’s disease or a neoplasm. A peptic ulcer–induced fistula to the colon is rare. Most cholecystocolic fistulas are secondary to cholecystolithiasis. Resulting inflammation and fibrosis, generally involving the hepatic flexure or proximal transverse colon, mimics the barium enema appearance of a primary colon adenocarcinoma.

One complication of interleukin-2 therapy is bowel perforation.

Pneumatosis Coli

Pneumatosis cystoides intestinalis is discussed in more detail in Chapter 4.

Pneumatosis coli represents pneumatosis cystoides intestinalis limited primarily to the colon. Similar to small bowel, pneumatosis coli can be subdivided into ischemic and nonischemic (benign) causes. It is characterized by multiple gas-filled cysts within bowel wall. Pneumatosis most often affects the left colon; a redundant sigmoid colon is a common ancillary finding. An occasional colonic intussusception is associated with pneumatosis cystoides intestinalis.

Pneumatosis coli is readily diagnosed with conventional radiography. A barium enema or CT confirms the intramural location for these gas collections. Endosonography reveals hyperechoic collections with acoustical shadowing.

Body Packer

Smuggling of cocaine or heroin concealed in the gastrointestinal tract is not new. The drugs are typically wrapped in cellophane or condoms and swallowed. In general, rupture of a single package is above the toxic dose and is fatal. At times these packages also obstruct the bowel. Most of these “mules” are treated conservatively, although an occasional one requires surgery.

Both US and conventional abdominal radiographs readily detect swallowed drug packages. Conventional radiographs and CT identify cannabis and cocaine packages as wellmarginated, rectangular, high-density structures surrounded by a gas halo, called the double condom sign; heroin packages are seen as poorly outlined structures resembling stool and are

ADVANCED IMAGING OF THE ABDOMEN

difficult to identify on conventional radiographs. Ultrasonography of cannabis packages reveals round hyperechoic structures.

Vascular Lesions (Bleeding)

Discussed here are those entities manifesting primarily by bleeding. Ischemic colitis has been discussed in a previous section.

The etiologies of rectal bleeding in adults are wide-ranging (Table 5.8) and differ between pediatric patients and adults; in pediatrics it is worthwhile to consider causes of rectal bleeding by age (Table 5.9).

Detection

Contrast-enhanced CT is at times worth-while in a patient with suspected lower gastrointestinal bleeding. Contrast extravasation is obviously diagnostic, but bowel wall contrastenhancement or presence of a focal lesion also point towards a bleeding site.

Technetium-99m–red blood cell scintigraphy is often employed as a screening examination for patients with suspected colonic bleeding. Should a nuclear medicine bleeding scan be obtained and mesenteric arteriography per-

Table 5.8. Etiologies of rectal bleeding in adults

Colonic Diverticulosis

Inflammatory bowel disease Ischemic colitis

Infectious colitis Neoplasms Hemorrhoids

Angiodysplasia/arteriovenous malformations Portal hypertension—colonic varices

Small bowel

Inflammatory bowel disease Angiodysplasia/arteriovenous malformations Diverticular causes

Meckel’s diverticulum related Neoplasms

Proximal to ligament of Treitz Peptic ulcer disease

Angiodysplasia/arteriovenous malformations Dieulafoy lesions

Portal hypertension—gastroesophageal varices

formed only if the scan is positive? The primary purpose of such a policy is to increase the percentage of positive arteriograms, yet one retrospective study concluded that a prior positive bleeding scan did not increase the odds of obtaining a positive angiogram (222), a finding at odds with a number of other studies. Thus arteriography in patients with suspected acute gastrointestinal bleeding detected bleeding in 22% of studies, but after instituting a protocol requiring positive scintigraphy before performing arteriography, the positive arteriography rate increased to 53% (223).

In patients with suspected acute lower gastrointestinal bleeding, Tc-99m–red blood cell scintigraphy achieves >80% sensitivity in detecting bleeding and in those with positive scan localizes a bleeding site in about 70%.

Colonoscopy can potentially identify a colonic bleeding site. A practical limitation exists if blood and blood clots obscure adequate visualization. Also, complete colonoscopy is necessary because in up to one third of patients a bleeding site is in the cecal region.

Therapy

After mesenteric angiography identifies a bleeding site, immediate therapeutic options include arterial embolization and infusion of vasopressin. Arterial embolization is viable therapy for most acute lower gastrointestinal bleeding. The clinical success of embolization, judged by no rebleeding, is achieved in about 90% of

patients. Even hemorrhage from a large vessel can be arrested. Postembolization ischemia is rarely an issue with this technique.

One refinement is superselective microcoil embolization, with embolization performed at the vasa recta or the marginal artery of Drummond level. In one study, bleeding was arrested on a long-term basis in over 80% of patients (224). Hemostasis can be expected in all except those with a dual blood supply to the bleeding site, yet even in the latter significant reduction of bleeding is achieved. Occasional bowel ischemia, rebleeding, and even infarction are recognized complications of this procedure. At times superselective embolization includes a combination of coils, polyvinyl alcohol, and gelatin sponge particles.

Diverticular Bleeding

A typical diverticular bleed tends to be massive and arterial, and it stops spontaneously. Often little other evidence of diverticulitis is present. Past teaching was that bleeding is due to erosion of a small artery overlying the diverticulum, but whether such erosions differ from a Dieulafoy lesion is not clear.

Right-sided diverticulosis tends to present with massive rectal bleeding more often than left-sided disease, yet in general, cecal bleeding is more difficult to control than more distant bleeding.

In patients without definitive therapy, recurrent hemorrhage occurs in about 10% at 1 year.

Angiodysplasia

Among a healthy, asymptomatic population prevalence of angiodysplasia is <1%. These ectatic veins, venules, and capillaries probably develop secondary to local degeneration, especially with aging. A deficiency of collagen type IV is found in mucosal vessels in angiodysplasia. They occur in both the small and large bowel. Most, however, are smaller than 10mm and are located in the right colon. They range from single to multiple. Most manifest in the elderly, although angiodysplastic hemorrhage does occur in young patients. Earlier reports suggested an association between angiodysplasia and aortic stenosis, but more recent studies do not confirm such a link.

Unusual associations of rectal and sigmoid colon angiodysplasia-like lesions include a 12- year-old boy with Klippel-Trenaunay-Weber syndrome who developed hematochezia (225). The presence of colon angiodysplasia, small bowel lymphoma,and duodenal carcinoid in the same patient suggests a more than fortuitous association (226).

Bleeding ranges from iron-deficiency anemia to a life-threatening acute hemorrhage.

Angiodysplasia is not detected by barium enema. Angiography and colonoscopy detect only some of these flat lesions. Most angiodysplasias are intramucosal in location, although an occasional one is deeper and thus not visible. If bleeding, scintigraphy is an appropriate first imaging modality employed.

If colonoscopy detects an incidental angiodysplasia in an otherwise asymptomatic individual, therapy probably is not necessary.

Selective mesenteric angiography reveals a tuft of abnormal vessels and an early filling vein. Superselective arterial embolization is common therapy to arrest acute bleeding from angiodysplasias. In some patients, estrogen-progesterone combination therapy has been successful in preventing rebleeding from angiodysplasias (227).

Arteriovenous Malformation

Most arteriovenous malformations are intramural in location. An occasional one has a polypoid appearance. Except for very small ones,

they are detected with contrast-enhanced imaging.

Varices

Most colonic varices are associated with portal hypertension. Why only some patients develop colonic varices is not clear, although the prevalence of these varices increases in those who have had prior transection and devascularization of esophageal varices, esophageal sclerotherapy, or thrombosis of coronary and azygous drainage veins. Congenital colon varices are rare; in the absence of portal hypertension resection of such varices is curative. Varices have developed secondary to mesenteric venous obstruction and, rarely, with splenic vein thrombosis. Also rare are idiopathic colonic and mesenteric varices.

Bleeding from colorectal varices can be massive; portal hypertension needs to be excluded in these patients.

Colonic and perirectal varices can be diagnosed with contrast-enhanced CT. What is surprising is that in some patients with portal hypertension, CT detects some pararectal varices not visualized by colonoscopy and vice versa; the inferior mesenteric vein is significantly larger in patients with rectal varices than in those without.

Portal Hypertensive Colopathy

Vascular ectasia-like lesions in the colon, called portal hypertensive colopathy, develop in some patients with portal hypertension. These lesions consist of numerous irregular vessels having a hyperemic “cherry-spot” appearance.

About one third of patients with severe cirrhosis have colonic wall thickening, predominantly in the right colon (228); most patients do not have symptoms referable to the colon, and these changes presumably are related to underlying portal hypertension. Clinically these patients range from asymptomatic, to recurrent rectal bleeding, to episodes of massive hemorrhage.

Transjugular intrahepatic portosystemic shunting (TIPS) does control bleeding from portal hypertensive colopathy and, in fact, the ectasia-like colonic lesions tend to disap-

COLON AND RECTUM

pear. Colopathy is also corrected after liver transplantation and correction of portal hypertension.

Juvenile Polyps

Juvenile polyps persist into adulthood. Occasionally such a juvenile polyp first manifests by a massive lower gastrointestinal bleed.

Dieulafoy Lesion

A small gastric mucosal defect associated with a bleeding submucosal artery is called a Dieulafoy lesion (these lesions are discussed in more detail in Chapter 2). Since their initial detection in the stomach, similar lesions have been found in the small bowel and colon. They appear to be more common in the proximal colon. They develop in both adults and children. Dieulafoy lesions occur in diverticula, where their differentiation, if any, from a typical diverticular bleed is problematic.

Dieulafoy bleeding is often massive but intermittent. Similar to gastric Dieulafoy lesions, colonoscopic sclerotherapy has successfully arrested bleeding. Selective mesenteric angiography differentiates Dieulafoy lesions from angiodysplasias—the former consists of a small bleeding artery.

Immunosuppression (AIDS)

In patients with human immunodeficiency virus infection, superimposed opportunistic infections are common. Yet not all symptoms and lesions are due to infection; some are neoplastic and others idiopathic.

Infection

Colonic histoplasmosis is not uncommon in patients with AIDS, with the most common site being the ileocecal region. Clinically, some patients develop a palpable tumor, and imaging reveals an annular constricting lesion. At times more diffuse involvement mimics ulcerative colitis. Colonoscopy in one patient revealed volcano-like ulcers and tumors; CT identified both a colon tumor and hypodense adenopathy (229); histology and biopsy cultures and lymph

node aspirate revealed Histoplasma capsulatum, but the patient eventually developed bowel obstruction and peritonitis. At times Gomori staining of colon biopsies aids in establishing this diagnosis.

Patients with AIDS are prone to C. difficile infection. Among other predisposing factors in this population is the common use of antibiotics; clinically, symptoms of C. difficile infection in AIDS patients tend to be more severe than in non-AIDS patients.

Cytomegalovirus colitis is common in this patient population and manifests as ulcers and submucosal hemorrhage, with these ulcers ranging from aphthous to relatively deep cavities surrounded by inflammation and edema. These findings are similar to those seen in pseudomembranous colitis. In some, cytomegalovirus infection results in a focal colonic tumor. Cytomegalovirus is diagnosed by finding viral inclusion bodies in colon biopsies. These inclusions are more common in cecal biopsies rather than more distally; therefore, complete colonoscopy is necessary.

Similar to other patients, AIDS patients develop pseudomembranous colitis when treated for an infection.

It should be kept in mind that AIDS patients also develop appendicitis.

Neoplasm

Although not common, HIV infection appears to play a role in colorectal cancer development. These patients are at risk for anal squamous and cloacogenic carcinomas.

An occasional rectal Kaposi’s sarcoma is successfully treated with radiation therapy.

Complications of non-Hodgkin’s lymphoma in these patients include duodenocolic fistula and intussusception.

Other

Some AIDS patients develop pneumatosis coli. Usually a late finding, in most patients, it is benign and clears spontaneously. The right colon is involved more often than the left. Occasionally pneumatosis coli is associated either with intraperitoneal or retroperitoneal gas and suggests a perforation, although bowel perforation is not detected.